1. Field of the Invention
The present invention relates to a material for contact lens and contact lens prepared from the material. More particularly, this invention relates to a material for contact lens obtained by visible ray polymerization and contact lens prepared from this material.
2. Description of Prior Art
In general, a material for contact lens consists of a polymer obtained by polymerization of one or more monomer(s). A contact lens is prepared by molding and processing this material. Thus, properties of the polymer, the material of the contact lens strongly influence on the quality of a contact lens.
Various kinds of materials for contact lens are known. For example, Japanese Patent Publication (KOKOKU) No. 59-33887 discloses a polymer obtained by polymerizing a monomer mixture containing acrylic acid in the presence of a photopolymerization initiator such as benzoin by wide-range ultraviolet ray irradiation. A non-water-containing soft contact lens is obtained by shaping and polishing the resulting polymer into the form of a contact lens and softening the polished lens by esterification and/or transesterification with an alcohol.
Japanese Patent Disclosure (KOKAI) No. 62-229113 discloses a polymer applicable to a material for contact lens. This polymer is obtained by thermal polymerization of a monomer mixture substantially containing fluorinated (meth)acrylate and acrylic acid or alkyl acrylate in the presence of a peroxide or an azo compound, or by ultraviolet ray polymerization of the above monomer mixture in the presence of photopolymerization initiator such as benzoin methyl ether. A contact lens is also obtainable from this polymer by shaping and polishing the polymer into the form of a contact lens and softening the polished lens by esterification and/or transesterification with an alcohol.
In this specification hereafter, (meth)acrylate means both of acrylate and methacrylate.
Since a contact lens is required to be transparent, and to be satisfied with certain properties such as optical properties and form stability, acrylate homopolymers and copolymers such as polymethyl methacrylate are typically used for a contact lens. However, it is hard to obtain acrylate (co)polymers by the heat polymerization method mentioned above. This is because acrylic acid is active to the polymerization reaction and the heat of polymerization is large. In fact, polymerization rate of acrylic acid is fast and exotherm of the polymerization reaction is large. As a result, the control of the polymerization is difficult and the resulting (co)polymer often foams. The foamed (co) polymer is not transparent and is unsuitable as a material for contact lens. Further, in the case where thus obtained (co)polymer is shaped into the form of contact lens and subjected to an esterification treatment with an alcohol to soften the lens, this (co)polymer sweels with the alcohol. The strength of the contact lens prepared from the (co) polymer obtained by the thermal polymerization is insufficient and the contact lens is easily destroyed by the swell.
Polymers obtained by ultraviolet ray polymerization do not have the above mentioned problems but have the other problems. That is, some of monomers used for polymerization or (co)polymers have a group absorbing ultraviolet ray such as an ester group and as a result, it is difficult to obtain a relatively thick (5 to 10 mm) material for contact lens. Additionally, a difference in polymerization state between the surface and the central of a contact lens material, so-called "polymerization strain" makes impossible to obtain a material with uniform properties. Since ultraviolet rays are utilized for the polymerization, it is required to use an aparatus from which ultraviolet rays do not come through during polymerization.